The present invention relates to a novel target material for use in the sputter formation of a metal silicide film in electrode wiring in a semiconductor device. The invention also relates to a process for producing such target material.
Recent years have seen the increasing use of films of high-melting metal silicides (e.g. MoSi.sub.2, WSi.sub.2, TaSi.sub.2 and TiSi.sub.2) in electrode wiring in semiconductor devices instead of the previously used films of aluminum, Al-Si alloys or polycrystalline silicon. Compared with these electrode wiring materials, high-melting metal silicides have low electric resistivities and high resistance to oxidation at elevated temperatures. Semiconductor devices using such metal silicides in electrode wiring are capable of faster arithmetic operation and can be fabricated with smaller chances of corrosion by chemicals and oxidation by treatments at elevated temperatures.
Films of high-melting metal silicides are conventionally formed by sputtering. Taking as an example molybdenum silicide (MoSi.sub.2) formed by reacting Mo with Si, high tensile stress will develop in the silicide film because the silicide has a smaller volume than the sum of the volumes of the individual reactants. The development of tensile stress can be reduced by using Si in excess over Mo, so it is preferred to provide a film composition, or the composition of the target material, such that the atomic ratio of Si to Mo is greater than 2. Another advantage of making a molybdenum silicide film having an excess amount of silicon is that the excess Si is oxidized to form a protective silica film on the film surface, making it compatible with the currently used silicon gate processing. However, if the composition of the target material is such that the atomic ratio of Si to Mo is greater than 4, the film made by sputtering this target material also has a Si/Mo (atomic ratio) greater than 4. This film however has an undesirably high sheet resistance. As will be apparent from the above description with reference to MoSi.sub.2, the films of high-melting metal silicides of the type intended for use in electrode wiring in semiconductor devices must have such a composition that 2&lt;Si/M' (atomic ratio)&lt;4 (M': high-melting metal). In order to form films satisfying this relation, the target material used in sputtering must be a composite of M'Si.sub.2 and Si and have such a composition that 2&lt;Si/M' (atomic ratio).ltoreq.4.
The target material meeting these two requirements is conventionally produced by first preparing a mixture of M' powder and Si powder so adjusted as to provide the desired target composition and then sintering the mixture by either the normal sintering method or the hot press method.
Production by such powder metallurgical techniques has one serious problem: because of oxygen being present in high volumes (.perspectiveto.13,000 ppm) in the powder mix, especially in the Si powder, the resulting target contains as much as about 2,500 ppm of oxygen and the film formed by sputtering this target also has a high oxygen content and exhibits an increased electrical resistance when it is used in electrode wiring in a semiconductor device. The oxygen in the Si powder assumes the chemical form of SiO.sub.2 and cannot be removed by ordinary methods.